Martin V takes three years on the journey south to Rome, moving cautiously between warring principalities and armies of condottieri. This is an Italy in which unscrupulous men are beginning to establish courts of glittering brilliance.

The pope newly crowned at Constance looks a tentative figure among such dangers, but over the following decades the papacy adjusts to the realities of Renaissance Italy. By the beginning of the next century unscrupulous popes have made Rome the most brilliant court of all.

The pope who begins the transformation of Rome, in the mid-15th century, has none of the scurrilous characteristics associated with the pontiffs of half a century later. He is Nicholas V, a scholarly man who founds the Vatican library, employing hundreds of scholars and copyists to provide the basis of a great collection of manuscripts.

The familiar image of a Renaissance pope begins a little later, with the election of Sixtus IV in 1471. His patronage of the arts is evident in the Sistine chapel and the Sistine choir, both named after him. But his lavish patronage goes hand in hand with a very worldly conduct of the Vatican's affairs.

Sixtus, a Franciscan friar from a poor family in the region of Genoa, brings the papal practice of nepotism to new heights. While greatly enriching his nephews (seven of whom he makes cardinals), he also uses them as his agents in the power politics of rival Italian states. The scheming of one nephew even results in the murder of one of the Medici in the cathedral at Florence during High Mass.

Another nephew learns his trade so well with Sixtus that he easily outdoes his uncle, both in politics and patronage, when he is elected to the papacy as Julius II.

Between the pontificate of Sixtus IV and of Julius II comes the most notorious of the Renaissance popes, Alexander VI. He manipulates Italian politics not with the help of nephews but through his son, Cesare Borgia (see the Borgias).

Alexander's successor Julius II is even more a man of his time. He is a pope who rides out in person to direct military campaigns, but he also commissions work from Raphael and Michelangelo. The frescoes of the Vatican and the Sistine chapel are created among the abuses which prompt the Reformation.

Julius II: 1503-1513

Pope Julius II, reigning from 1503 to 1513, represents the best and the worst of the newly self-confident Rome of the Renaissance. His energetic sense of purpose on behalf of the holy see emerges as naked aggression in territorial matters; yet the same boldness and ambition makes him the greatest patron of the arts in the history of the papacy.

In 1503, his first year in office, Julius launches the great scheme to rebuild St Peter's. In 1509 the pope invites Michelangelo to paint the ceiling of the Sistine Chapel and persuades Raphael to decorate three rooms in the Vatican. Christian Rome's greatest glories have been conceived within a space of six years.

St Peter's: 1506-1590

In April 1506 Julius II and his architect, Bramante, are ready to lay the foundation stone of the new St Peter's. A commemorative medal is struck with the classical inscription Templi Petri Instauracio (Renewal of the Temple of Peter), showing a view of a great domed basilica with a classical portico.

In spirit - though not in detail - this design is similar to the church which is eventually completed in 1590, by which time Raphael and Michelangelo and several others have succeeded Bramante as official architect for the scheme.

The final appearance of the exterior, particularly in its upper half, owes more to Michelangelo than anyone else. The church's magnificent profile, visible from miles around as an imposing architectural statement, inspires the age of the dome in western architecture - a natural choice for religious and public buildings during the next three centuries.

The completion date of the exterior of St Peter's, in 1590, aligns it perfectly for a role in another new architectural style. The large space in front of St Peter's, and the central features of its interior, are entrusted to Bernini. They become seminal examples of the baroque.

Bernini and baroque Rome: 17th century

In the transformation of Rome into a baroque city, no one plays a part comparable to that of the sculptor and architect Giovanni Lorenzo Bernini. In 1629 he is appointed architect to St Peter's, the creation of which has given a new excitement and dignity to the ancient city. Over the next forty years he provides magnificent features to impress the arriving pilgrims.

The first, completed in 1633, is the vast bronze canopy held up by four twisting columns (profusely decorated with the Barberini bees, for the pope at the time is Urban VIII). This structure, known as the Baldacchino, is at the very heart of the church - above the tomb of St Peter and below the dome.

The Baldacchino rises above an altar at which only the pope conducts mass. Visible between the columns, from the point of view of the congregation, is Bernini's other dramatic contribution to the interior of St Peter's. This is a golden tableau, a piece of pure theatre, above the altar at the far end of the church. Its central feature is the papal throne of St Peter, held aloft among the clouds.

Sculpted golden rays stream up from St Peter's throne towards heaven. In an extra dimension to the illusion they are joined by real rays of golden light, shining from the afternoon sun through an amber window in which the holy dove spreads his wings. This glorious blend of sculpture and architecture is achieved between 1657 and 1666.

During these same years Bernini's great contribution to the exterior of St Peter's is also under construction. The open space in front of the church, where pilgrims gather to hear the pope's Easter address, needs to be enclosed in some way to form a welcoming piazza.

Bernini achieves a perfect solution in the form of an open curving colonnade. The four concentric rows of columns provide covered walkways and a shape for the piazza, but they do so without closing it in - for there is no back wall. Meanwhile the balustrade above the columns is an ideal pedestal for the gesticulating stone saints who are an indispensable part of monumental baroque.

Baroque Rome perfectly reflects the mood of the Catholic Reformation. The city of the popes (whose temporal rule in central Italy and whose spiritual authority over the greater part of Christendom are now alike restored) is also the headquarters of the popes' energetic new missionaries, the Jesuits.

The sumptuous central church of the Jesuits - the Gesù, completed in 1575 - is an early and influential example of the baroque style. Its sculptural altars and painted ceilings, in which saints destroy heresy or fly heavenwards with flamboyant certainty, leave the visitor in no doubt that Rome and its brand of Christianity have recovered their confidence.

Classical Rome: 18th century

In the more leisurely mood of the 18th century, baroque Rome acquires a stream of wealthy and influential visitors who come here mainly for the city's pre-Christian past. It is the destination of young noblemen arriving from northern Europe, and particularly from Britain, on the Grand Tour. They pose here for their portraits against a backdrop of vases, columns and distant vistas. The very rich take home excavated fragments of classical sculptures, described as 'marbles'. Others make do with paintings, prints, or even cork replicas of famous ruins.

This period of settled calm, unusually long in Rome's history, is brought to an abrupt end by Napoleon.